CHARLESTON, W.Va. — A Columbia Gas Transmission line had thinned to nearly a third of the thickness it ought to have been before it exploded Tuesday near Sissonville, the National Transportation Safety Board said Thursday.

“At some locations, along the thinnest part of the pipe, there was approximately a 70 percent loss in the wall thickness,” NTSB member Robert Sumwalt told reporters. “To put it another way, in these locations, the pipe was only about 30 percent as thick as it should be.”

To be precise: a 6-foot segment along the bottom of the ruptured 20-inch diameter line was just .078 inches thick in some places, Sumwalt said. That’s “thinner than designed,” Sumwalt said. The pipe was permitted to handle 1,000 pounds of pressure per square inch of natural gas.

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Columbia is a subsidiary of Indiana-based NiSource.

Tuesday’s blast filled the sky with fire, scorched the earth and ruined a segment of Interstate 77. Amazingly, it did not seriously injure or kill anyone near Sissonville, a town of 4,000, or anyone on the busy stretch of highway.

The NTSB, which investigates incidents along interstate pipelines, is also trying to figure out if Columbia reacted quickly enough to the explosion, which occurred about 12:41 p.m. Tuesday.

At about that time, records indicate the pipeline experienced a drop in pressure. It was unclear Thursday if the drop was a possible cause or merely an effect of the rupture.

Investigators have already said no alarms went off in the company’s control center, which is situated several floors up in the Columbia building along the Kanawha River in Charleston. The control center is essentially the brains of a sophisticated natural gas operation.

It remains unclear how Columbia learned of the explosion.

“I don’t know if Columbia Gas knew it was their line before they were called,” said Dale Petry, director of Kanawha County emergency management.

A Columbia spokeswoman and other Columbia staffers either declined to comment, did not return calls or did not provide details.

Kanawha County Metro 911 logs indicate the company may not have immediately known what was happening in Sissonville.

Dispatchers were still trying to notify Columbia about the explosion at 1:14 p.m., or more than a half hour after it occurred.

At 1:20 p.m., dispatchers learned that Columbia employees were in the process of shutting gas lines off at the Lanham Compressor Station near Cross Lanes. The station, which pressurizes gas so it flows steadily through pipes, is part of Columbia’s west-to-east SM-80 pipeline system. The line that exploded is connected to the station. Another explosion occurred near the station in 2002.

Kanawha County Manager Jennifer Sayre praised what she saw as the company’s quick reaction. By the time she was on the scene — about a half hour after the first call, she said — the company was already there.

“It did take about an hour to get it shut off, but they were on scene immediately,” she said.

It took just over an hour for Columbia to manually stop the flow of gas to the pipe.

Companies that use automatic shut-off valves can stop the flow of gas “instantaneously, if not very, very quickly” during an emergency, Sumwalt said earlier this week.

NTSB investigators are interviewing the control center’s staff this week about the alarm and about the timeline of events. Results of the investigation will not be made public for months.

In the meantime, Columbia’s SM-80 line remains shut off, according to NTSB.

“We want to make sure that this pipeline is not affected in adverse ways in other areas,” Sumwalt said.

But it will be the U.S. Pipeline and Hazardous Materials Safety Administration, or PHMSA, that will determine when and if Columbia can re-open SM-80.

Pipelines are regulated by a few agencies, including PHMSA and the state Public Service Commission. Concerns have been raised about staffing levels at the federal agency, which has just over 200 full-time pipeline safety staff in a country with 2.5 million miles of pipelines.

In West Virginia, five Public Service Commission staffers regulate 4,000 miles of transmission line like the kind that ruptured Tuesday. Transmission lines are the largest of the natural gas lines.

Following a deadly pipeline explosion in California in 2010, that state’s utility regulator hired an outside consultant to do an independent review of not only the company that owned the pipeline but the regulators themselves.

In West Virginia, companies generally police themselves and they have their work checked by state regulators. TheWest Virginia state regulators also do spot checks of certain elements of the pipeline.

The California independent review raised question about the use of “spot checks.”

“This can be an effective tool in assuring regulatory compliance, but less effective in achieving improvement in public safety and system integrity,” the California review found.

West Virginia PSC spokeswoman Susan Small said the PSC was awaiting the NTSB’s findings.

Sumwalt said there are “many things” that can cause pipeline thickness to deteriorate.

NiSource Vice President Jimmy Staton told Kanawha County legislators Wednesday that the ruptured pipe near Sissonville was laid in the 1990s.

“He said the vintage of it was 1990s and they were eager to find out why a pipe that young failed in that manner as well,” said Delegate Patrick Lane, who attended the Wednesday meeting.

If that’s the case, the pipe that ruptured was significantly younger than the vast majority of Columbia’s national pipeline system. About half of the lines predate 1960.

About 8,800 miles of Columbia’s nearly 12,000 miles of interstate pipelines were installed before the creation of federal pipeline safety standards, according to a recent filing the company made with the Federal Energy Regulatory Commission,

The company also says it has more “high risk” interstate pipe — about 1,100 miles of it — in the ground than any other gas company in the country.

The company wants to spend more than $300 million a year for as many as 15 years to deal with some of its highest-risk pipes and other facilities. It costs about $600,000 to totally replace a mile of transmission line, according to a recent study by financial analysts at Moody’s.

While its SM-80 line remains shut off and under investigation, Columbia is redirecting gas that would have flowed through the now-ruptured line, according to BENTEK Energy, a Colorado-based gas market analyst.

The PSC’s Small said David Hippchen, the state’s head of pipeline safety, also had questions about the failure of alarms to sound in Columbia’s control room.

“He said, ‘If we were conducting the investigation, we would be asking that question,’” Small said on Hippchen’s behalf.